They really waited until they had no choice: they refused to hear all cases until there was a circuit split, then they had no choice but to resolve the split.
RGB specifically addressed why the supreme court wasn't previously hearing marriage cases at that time - they don't generally hear cases when the circuit courts are in agreement. Once the circuit split then they addressed the issue, but only then. In fact the dissent in DeBoer v. Snyder (which is one of the four cases that was combined for the Supreme Court) said "Because the correct result is so obvious, one is tempted to speculate that the majority has purposefully taken the contrary position to create the circuit split regarding the legality of same-sex marriage that could prompt a grant of certiorari by the Supreme Court and an end to the uncertainty of status and the interstate chaos that the current discrepancy in state laws threatens."
